Heretofore there has been known a lens meter provided with a projection light source for projecting a measuring beam onto a test lens and a light receiving element for receiving the measuring beam which has passed through the test lens, the lens meter measuring optical characteristics of the test lens on the basis of a position in which the light receiving element receives the measuring beam after passage through the test lens.
According to this conventional lens meter, with the test lens set in a lens receptacle, the measuring beam is projected onto the test lens and optical characteristics of the test lens such as spherical degree S, cylindrical degree C, axial angle A, and prism quantity Pr, at central and peripheral portions of the test lens are measured.
In this conventional lens meter, when optical characteristics are to be measured at the peripheral portion other than the central portion of the test lens, the test lens is moved while allowing the back of the lens to follow the lens receptacle .
In this connection, consideration will now be given about a spectacle lens-worn state. As shown in FIG. 1, when a person tries to see far, the man mainly uses a light ray P1 which passes through a central position of a spectacle lens 1, but for seeing near, the person uses a light ray P2 which passes through a peripheral position of the spectacle lens 1 and the light ray incident on the lens 1 is oblique to a back 1a of the spectacle lens 1. In FIG. 1 being referred to, the reference numeral 2 denotes an eyeball and the reference mark OL denotes a center of a turning movement of the eyeball 2.
In the conventional lens meter, as shown in FIG. 2, a test lens 3 is measured at a peripheral position thereof while allowing a back 3a of the lens to follow a lens receptacle 4. Thus, optical characteristics of the test lens 3 are measured at a peripheral position under the condition that a measuring beam P3 is incident perpendicularly to the back 3a of the lens 3. Therefore, the optical characteristics measured at a peripheral position of the test lens 3 by the conventional lens meter are not always considered to reflect an actual spectacle lens-worn state.
Particularly, today, with the advent of an aging society, the use of a progressive multi-focus lens is increasing, and in spectacle shops the opportunity of measuring optical characteristics of a progressive multi-focus lens is increasing. In the progressive multi-focus lens, the measurement is made for at least a far-sight portion and a near-sight portion and it is necessary that optical characteristics of the test lens 3 be measured at various positions. However, if the way of light ray passage in an actual spectacle lens-worn state and the way of light ray passage in the measurement using the conventional lens meter are different from each other, it is difficult to exactly evaluate optical characteristics of the progressive multi-focus lens.
There has been proposed a construction in which a turning bench adapted to turn around a center of a turning movement of an eyeball is attached to a lens meter different in measurement principle from the above lens meter.
FIG. 3 shows an example of an optical system in such a lens meter. In the same figure, the reference numeral 1A denotes an LED as a projection light source for the projection of a measuring light beam, 1B denotes a projection lens, 1C denotes a relay lens, 1D denotes a light receiving lens, and 1E a light receiving element, with a measurement target 1F being disposed between the projection lens 1B and the relay lens 1C. For example, four LEDs 1A are disposed around a measuring optical axis OA. A disposed plane 1G of the LED 1A and a disposed position 1H of the test lens are conjugate with respect to the projection lens 1B and the relay lens 1C. The measurement target 1F and the light receiving element 1E are conjugate with respect to the light receiving lens 1D when the test lens 1L is not disposed in the position 1H. As shown in FIG. 3(a), a measuring beam which passes through a hole formed in the measurement target 1F converges to a single spot on a light receiving surface of the light receiving element 1E.
When the measurement target 1F lies in a reference position 1J shown in FIG. 3(a), if the test lens 1L is set to its disposed position and a measuring beam is projected onto the test lens 1L from a back side (the side which faces an eye in a spectacle lens-worn state), there occurs a deviation in the conjugate relation between the measurement target 1F and the light receiving lens 1D.
In the conventional lens meter, for eliminating this inconvenience, as shown in FIG. 3(b), the measurement target 1F is moved along the measuring optical axis OA so that the measurement target 1F and the light receiving element 1E become conjugate, and optical characteristics are measured on the basis of the amount of movement .DELTA.t of the measurement target 1F. In this lens meter of an old type, however, not only the optical path of the optical system is long, but also the structure is complicated because a turning bench is attached to the lens meter.
It is an object of the present invention to provide a lens meter of a simple structure capable of measuring optical characteristics which reflect a spectacle lens-worn state to the utmost extent.